Glutamate metabolism revisited: Clarifying the pathophysiology of GLS mutations

Abstract

Inborn Errors of Metabolism (IEMs) are a class of inherited genetic disorders caused by variants in genes coding for proteins that function in metabolism. The research presented in this thesis describes the identification and disease mechanisms of two novel IEMs caused by mutations in the GLS gene. This gene encodes the protein glutaminase, which converts the amino acid glutamine into glutamate. Glutamine is a source for many proteins and is important for ammonia detoxification. Glutamate is an important neurotransmitter and plays a pivotal role in energy metabolism and redox homeostasis. Excess of these amino acids however can be harmful. A balance between these amino acids is therefore essential. In chapter 1, the required steps for the identification of new IEMs are elaborated. Furthermore, it is exemplified how the development of genetic and analytical techniques improved our ability to discover the genetic cause of IEMs. In chapter 2, we describe a new IEM in 4 children of 2 unrelated families, caused by bi-allelic loss-of-function mutations in GLS. This leads to neonatal, respiratory insufficiency, epilepsy and death. This was achieved through a “genome first approach”, in which Whole Exome Sequencing identified the variants in the GLS gene, which co-segregated within the family, and targeted metabolic analyses confirmed loss-of-function. In chapter 3, we describe another novel IEM caused by a GLS mutation. This mutation caused GLS hyperactivity, resulting in low glutamine and high glutamate levels. This patient had early onset cataract diagnosed at age 3 months, dermatological abnormalities, axial hypotonia and profound developmental delay. In a cell model in which the GLS hyperactivity mutation was expressed, we showed that metabolic compensatory mechanisms were activated: glutaminase protein expression was decreased, while glutamine synthetase (the reciprocal enzyme) protein expression was increased. We furthermore demonstrated that redox capacity buffer was decreased. We demonstrated the causality of GLS hyperactivity for cataract formation, as zebrafish with GLS hyperactivity developed cataract, which could be alleviated with GLS activity inhibition. To visualize the lens of the zebrafish embryo (smaller than 0.1mm) and lens-opacities, we developed a fluorescence microscopic method based on UV-Illumination (FUVI) in chapter 6. In chapter 4 the extensive downstream metabolic consequences of GLS hyperactivity were examined by untargeted metabolomics. This method provides an extensive “metabolic-fingerprint”. It is less specific than targeted metabolic techniques, but enables the detection of thousands of metabolites with one test. We demonstrated that GLS hyperactivity not only affected glutamine and glutamate levels, but that it has far-reaching downstream metabolic consequences. GLS loss-of-function and GLS hyperactivity are not the only inborn errors of glutamate metabolism. In chapter 5 we review all reported inborn errors of enzymes of glutamate metabolism and provide an overview of their clinical and biochemical characteristics. By drawing phenotypic and biochemical parallels, we attempt to create insight into the clinical effect of disturbed glutamate metabolism. By drawing parallels between phenotypic features of these patients, we showed that neurological, ophthalmological and dermatological features were often exhibited. This provides us with knowledge about the importance of regulated glutamate homeostasis for these tissues.